Coupling of single-mode optical fibers is of particular interest in applications requiring devices interposed between the fiber ends for processing the optical signal, including modulators, beam splitters, Bragg's cells, etc.
As is known, under these conditions the transmission of the light radiation from a single-mode optical fiber to another involves a number of difficulties if very low coupling losses are desired. That is due to the fact that accepted tolerances in displacement errors are of the order of microns and those allowed in axial misalignment errors are of the order of this degree. Hence the difficulty encountered both in carrying out the correct fiber positioning and in maintaining the proper position is evident.
The solutions used heretofore require the use of coupling devices with high-precision guides, in order to obtain the alignment between lenses and fibers without further adjustments.
According to other solutions, fiber ends to be coupled are positioned with high precision by micromanipulators and then are kept in the selected position with adhesives or solders. These methods still have a number of difficulties in their application owing to adhesive shrinkage during a drying phase, which causes renewed axial displacement and misalignment of the juxtaposed fibers.
The coupling problem between a collimated beam source and a single-mode fiber has already been solved by Newport Corporation, which has presented in its 1985 catalog a device referred to as a "High-Precision Steering Lens Single Mode Fiber Coupler". This device basically consists of a biconcave and a biconvex lens, which are traversed by the collimated beam outgoing from the source. The biconcave lens is used to obtain beam deflection, while the biconvex lens is used to focus the beam on the fiber-end face. The biconcave lens can be translated in a plane perpendicular to the beam, obtaining a corresponding translation of the image focused on the fiber. The ratio between these translations is equal to the ratio between lens focal distances, that is why small image displacements can be obtained through much greater biconcave-lens displacements.
A device of this type along is not able to implement coupling between two fibers. In fact if the two fibers are not well positioned, the beam focused on a fiber does not easily remain within its acceptance angle, even if the core image of the optical fiber is correctly positioned on the other fiber core.
This problem could be solved by having one device of the mentioned type for each fiber. However, the further degree of freedom obtained would be detrimental both to the simplicity of device implementation and to the ease of coupling adjustment.